This invention relates to a power delivery system having a continuously variable ratio transmission and, more particularly, to a control system and a control method for such a system, such as might be used in an automotive vehicle.
The quest for greater fuel economy of automotive vehicles has led to significant improvements in engine and transmission design and control. Continuously variable ratio transmissions (CVT) have shown particular promise in this regard. It will be appreciated that at any given vehicle speed, and for any needed propulsive force, a certain transmission ratio will provide maximum fuel economy for a given engine. In addition, for any given vehicle speed, one transmission ratio will permit maximum acceleration with that engine. Since a CVT with the proper ratio range can provide any desired transmission ratio, it is obviously attractive for automobiles from the standpoint of economy, low emissions and performance. If the mechanical efficiency of the CVT is high and its ratio range is wide enough, it can even be possible to have both maximum economy and maximum performance in the same vehicle. Among the obvious benefits are fully automatic operation, smooth, stepless and rapid response to driver demand, and quieter cruising.
Many different CVT configurations have been developed in the prior art. These include, for example, hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multi-speed gear boxes with slipping clutch; and V-belt traction drives. Of these the V-belt traction drives appear attractive for small to medium size passenger car applications because of their compactness, lightness and simplicity of design. Basically, this type of CVT comprises a V-belt which interconnects a driver sheave and driven sheave, the diameters of the sheaves being variable to change the ratio of the CVT. Recent advances in belt design have resulted in improved belt durability and longevity. If sheave movement can be properly controlled so as to avoid undue stresses on the belt, it is expected that a very long belt life can be achieved.
Many control schemes have been devised for engine-CVT systems in attempts to maximize fuel economy. These have been based on empirical analyses of individual engine performance, and the realization that, for any desired power output, there is an optimum combination of engine speed and torque which will result in minimum fuel consumption. This is illustrated in FIG. 2.
FIG. 2 is a typical performance map of a four cylinder spark ignition passenger car engine having a displacement of approximately 2.5 liters. The map is a plot of engine torque T.sub.E and brake horsepower BHP as a function of engine speed N.sub.E. The dot-dash line near the top of the map is a plot of engine torque at full throttle. The series of curves in solid black lines are fuel consumption contours, indicating constant brake specific fuel consumption (BSFC) in lb.M/BHP-hr. Minimum fuel consumption occurs at a point designed by 0.4 pounds per horsepower-hour. The series of dashed lines indicate power output of the engine. The ideal operating line for low fuel consumption is indicated by the heavy solid line f(N.sub.E), this curve being a function of engine speed. The ideal operating line for low fuel consumption is purely a function of engine characteristics and is optimal regardless of vehicle road speed. Other ideal operating lines may appear on the performance map, for example, the ideal operating line for low emissions.
In a vehicle with a conventional, manually shifted gearbox, forward speed ratios usually are available in only four or five steps. The operating point of the engine on the performance map is determined by drive shaft speed, power or torque commanded, and transmission gear ratio. Since there are only a few gear ratios available in a typical transmission, the engine must be throttled much of the time. The engine must therefore operate most of the time at high BSFC values. In contrast, a CVT is able to vary its speed ratio continuously to allow the engine to run at wider throttle and lower BSFC values.
Perhaps the most difficult task demanded of a control system for an engine-CVT system is to maintain engine operation along the ideal operating line. This is due to the almost continuous transient nature of operation of an automotive vehicle, there being hardly ever a time when road load and commanded torque or power remain constant. Transient conditions usually are dealt with by a change in CVT ratio, engine speed and throttle. Prior art control systems, by their very nature, permit an excursion of engine operation away from the ideal operating line before returning back to it at steady state. An example of such an excursion is shown in FIG. 2 by dashed line X-Y-Z. The result is that engine operation approaches, but hardly ever is maintained on the ideal operating line. Two such prior art systems are illustrated in FIGS. 3 and 4.
FIG. 3 schematically illustrates a system devised by Peter Stubbs for British Leyland. This system is described in greater detail in Stubbs, "The Development of a Perbury Traction Transmission for Motor Car Applications", ASME Publication, No. 80-C2/DET-59 (August 1980). In this system, engine speed (N.sub.E), throttle (ET) position (.theta.) and CVT ratio (e) signals are all fed to a computer controller which has, in its memory, the engine operating characteristic for minimum fuel consumption. The computer controller generates, as a function of these variables, an engine control signal (Nc) for adjusting the position (.theta.) of the throttle, and a ratio rate signal (Ec) which changes the ratio of the CVT. The throttle is under the direct control of the signal (.alpha.) from the vehicle accelerator pedal so that, while the engine control signal may vary the throttle position somewhat from that commanded by the driver, the throttle position still is primarily a function of commanded power or torque.
FIG. 4 is a schematic representation of the system devised by Miyao for Aisin Seiki Co., Ltd., Japan, This system is described in greater detail in U.S. Pat. No. 4,091,690. Here, as in the Stubbs system, engine throttle is primarily a function of commanded power or torque by direct connection with the accelerator pedal. The computer generates a ratio rate signal (Ec) to change the CVT ratio (R) as a function of measured throttle position (.theta.) and engine torque (Te) and speed (Ne). Inherently sensed output torque (To) also affects the CVT ratio. Indicated by Re is the resistance that the vehicle experiences in traveling, such as road load.
In these, as well as in virtually all other engine-CVT control systems, throttle position is controlled directly by the vehicle accelerator pedal, or is a direct function of pedal position, as well as other parameters. Engine and transmission control usually are directly related to one another. Such control schemes permit engine operation during transients to vary from the ideal operating line. Excursions away from the ideal operating line result in less than optimum engine operation (e.g., excessive fuel consumption, or excessive emissions), until effective control is resumed by the system during steady state operation. As pointed out, earlier, however, most vehicular operation is transient in nature, rather than steady state, so that substantially all engine operation occurs off the ideal operating line. Emissions calibrations must therefore be made in a substantial portion of the engine performance map. Most prior art control systems also must be specifically tailored to particular engines. This requires numerous specially designed control systems for a fleet of differently powered vehicles. In addition, most prior art control systems cannot compensate for varying engine conditions, the result being vehicle driveability which varies with engine temperature, state of tune, age and altitude. Close duplication of conventional vehicle characteristics also is a problem with prior art CVT control schemes.
However, Japanese Patent Laid-Open No. 34057/1984 corresponding to U.S. Pat. No. 4,458,560 discloses a system which controls the transmission of an automobile completely independent of the control over the engine. This makes it easy to operate the engine along an ideal operating line. That is, the position of the engine throttle is completely independent of the position of the accelerator pedal. The position of the throttle, hence the torque delivered by the engine, is only a function of the rotational speed of the engine. This function can be modified to a desired form or relation, e.g., an ideal operating line for low fuel consumption, an ideal operating line for low emission of exhaust gas, and a composite ideal operating line for achieving both low fuel consumption and low emission of exhaust gas. The torque, horsepower, and other desired parameters of performance which are instructed by the accelerator pedal control the CVT ratio. The rotational speed of the engine depends on the load applied to the engine. This load is a function of the load applied by the road and, at the same time, a function of the CVT ratio. Accordingly, the position of the throttle is accurately adjusted according to the ideal functions whatever load is applied to the engine.
The present inventor has already proposed a control system in Japanese Patent Application No. 182543/1983 corresponding to U.S. patent application Ser. No. 06/656,776, still pending. This proposed system is based on the control system described just above and controls the engine and the CVT ratio in such a way that the engine always operates along an ideal operating line. When the clutch is engaged, the opening of the throttle is smoothly varied according to the lowest fuel consumption.
The fundamental control system uses signals indicative of the torque, the horsepower, and other desired parameters of performance as input signals to control the CVT ratio, the parameters being instructed by the accelerator pedal. Especially, the pressure servo in a driven member of the CVT is controlled according to the position of the accelerator/the velocity of the driving shaft (.alpha./Nd.sub.s) The system proposed by the present inventor in the aforementioned Japanese Patent application No. 182543/1983 corresponding to U.S. patent application Ser No. 06/656,776 adopts the same principle. According to this control system, when the position of the accelerator pedal is high and the velocity of the driving shaft is low, the pressure P.sub.2 delivered to the pressure servo in the driven member is high. Thus, the diameter of the sheave in the driven member is large, making the transmission gear ratio (R=N.sub.e /Nd.sub.s) large. As a result, a large accelerating force is obtained to start the vehicle. On the other hand, when the position of the accelerator pedal is low but the velocity of the driving shaft is high, the pressure P.sub.2 delivered to the pressure servo in the driven member is low, making the diameter of the sheave in the driven member small. Then, the transmission gear ratio R assumes a small value, whereby the vehicle goes to steady-state running. Accordingly, this control system can be said to conform to the running condition.
However, this control system introduces the following problem when the driver releases the accelerator pedal to decelerate the running vehicle. As soon as the pedal is released while the vehicle is traveling, the pressure P.sub.2 delivered to the pressure servo in the driven member drops to zero, making the diameter of the sheave in the driven member smaller. Then, the belt is shifted to a higher position. Under this condition, if the velocity drops nearly to a velocity region in which the clutch is disengaged, the engine load becomes excessive, causing knocking. This may eventually bring about engine stall. Further, when the driver tries to restart the vehicle, the engine load is excessive, because the belt is situated on the higher side. Hence, it is difficult to start the vehicle.
This control system introduces another problem when the driver releases the accelerator pedal while the vehicle is traveling, often for obtaining engine brake. However, a sufficient amount of engine brake is not provided.